Neonatal Necrotizing Enterocolitis (NEC) is the most serious gastrointestinal disorder of preterm infants. With undefined mechanisms of pathogenesis, management of NEC is largely empiric, highly expensive, and often results in long term morbidity. The PI has discovered that loss of natural microbial balance in the preterm intestine is key to the development of NEC. Studies from his laboratory have shown that Gram (plus) organisms (enterococci/staphylococci) isolated from healthy infants can block Gram (minus) adherence and NEC-like disease in experimental models. Lactobacilli and bifidobacteria have been used in adults and children, including preterm infants, for prevention/treatment of a variety of gastrointestinal disorders. Recent data from his laboratory show that there is an almost complete lack of such harmless bacterial strains in the preterm intestine. Using identical experimental systems, data from his laboratory now indicate that certain strains of lactobacilli have similar potential for blocking NEC-like disease. In the current proposal, he plans to test the protective efficacy of a panel of lactobacilli/bifidobacteria in cell culture and animal models of NEC. In phase 2 he can immediately go to toxicity testing followed by a multicenter efficacy trial to ascertain the role of these bacteria in the prevention of NEC. If successful, for the first time lyophilized strains of such protective bacteria will be available to be marketed as a preventative therapy of this devastating disease.